diff --git a/Makefile b/Makefile
index 492eebe..1eb7777 100644
--- a/Makefile
+++ b/Makefile
@@ -5,9 +5,9 @@ SHARED_LIB = libsparsemap.so
 
 LIBS =  -lm
 #CFLAGS = -Wall -Wextra -Wpedantic -Of -std=c11 -Iinclude/ -fPIC $(LIBS)
-#CFLAGS = -Wall -Wextra -Wpedantic -Og -g -std=c11 -Iinclude/ -fPIC  $(LIBS)
+CFLAGS = -Wall -Wextra -Wpedantic -Og -g -std=c11 -Iinclude/ -fPIC  $(LIBS)
 #CFLAGS = -DSPARSEMAP_DIAGNOSTIC -DDEBUG -Wall -Wextra -Wpedantic -O0 -g -std=c11 -Iinclude/ -fPIC $(LIBS)
-CFLAGS = -DSPARSEMAP_DIAGNOSTIC -DDEBUG -Wall -Wextra -Wpedantic -Ofast -g -std=c11 -Iinclude/ -fPIC $(LIBS)
+#CFLAGS = -DSPARSEMAP_DIAGNOSTIC -DDEBUG -Wall -Wextra -Wpedantic -Ofast -g -std=c11 -Iinclude/ -fPIC $(LIBS)
 #CFLAGS = -Wall -Wextra -Wpedantic -Og -g -std=c11 -Iinclude/ -fPIC $(LIBS)
 #CFLAGS = -Wall -Wextra -Wpedantic -Ofast -g -std=c11 -Iinclude/ -fPIC $(LIBS)
 #CFLAGS = -DSPARSEMAP_DIAGNOSTIC -DDEBUG -Wall -Wextra -Wpedantic -Og -g -fsanitize=address,leak,object-size,pointer-compare,pointer-subtract,null,return,bounds,pointer-overflow,undefined -fsanitize-address-use-after-scope -std=c11 -Iinclude/ -fPIC $(LIBS)
@@ -39,16 +39,16 @@ $(SHARED_LIB): $(OBJS)
 
 examples: $(STATIC_LIB) $(EXAMPLES) $(TEST_OBJS)
 
-soak: tests/soak.c
-	tests/soak
-
 mls: examples/mls
 
-test: $(TESTS)
+tests: $(TESTS)
 
-check: test
+test: tests
 	env ASAN_OPTIONS=detect_leaks=1 LSAN_OPTIONS=verbosity=1:log_threads=1 ./tests/test
 
+soak: tests
+	env ASAN_OPTIONS=detect_leaks=1 LSAN_OPTIONS=verbosity=1:log_threads=1 ./tests/soak
+
 tests/test: $(TEST_OBJS) $(LIB_OBJS) $(STATIC_LIB)
 	$(CC) $^ -o $@ $(TEST_FLAGS)
 
@@ -76,19 +76,19 @@ examples/%.o: examples/%.c
 	$(CC) $(CFLAGS) -c -o $@ $^
 
 examples/ex_1:  $(LIB_OBJS) examples/ex_1.o $(STATIC_LIB)
-	$(CC) $^ -o $@ $(CFLAGS) $(TEST_FLAGS)
+	$(CC) $^ -o $@ $(TEST_FLAGS)
 
 examples/ex_2: $(LIB_OBJS) examples/ex_2.o $(STATIC_LIB)
-	$(CC) $^ -o $@ $(CFLAGS) $(TEST_FLAGS)
+	$(CC) $^ -o $@ $(TEST_FLAGS)
 
 examples/ex_3: $(LIB_OBJS) examples/ex_3.o $(STATIC_LIB)
-	$(CC) $^ -o $@ $(CFLAGS) $(TEST_FLAGS)
+	$(CC) $^ -o $@ $(TEST_FLAGS)
 
 examples/ex_4: $(LIB_OBJS) examples/ex_4.o $(STATIC_LIB)
-	$(CC) $^ -o $@ $(CFLAGS) $(TEST_FLAGS)
+	$(CC) $^ -o $@ $(TEST_FLAGS)
 
 tests/soak: $(LIB_OBJS) tests/soak.o $(STATIC_LIB)
-	$(CC) $^ -o $@ $(CFLAGS) $(TEST_FLAGS)
+	$(CC) $^ -o $@ $(TEST_FLAGS)
 
 todo:
 	rg -i 'todo|gsb|abort'
diff --git a/include/sparsemap.h b/include/sparsemap.h
index 571f431..65d4245 100644
--- a/include/sparsemap.h
+++ b/include/sparsemap.h
@@ -263,8 +263,10 @@ void sparsemap_scan(sparsemap_t *map, void (*scanner)(sm_idx_t vec[], size_t n,
  *
  * @param[in] map The sparsemap reference.
  * @param[in] other The bitmap to merge into \b map.
+ * @returns 0 on success, -1 and sets errno to ENOSPC when the merge might
+ * require more space than available.
  */
-void sparsemap_merge(sparsemap_t *map, sparsemap_t *other);
+int sparsemap_merge(sparsemap_t *map, sparsemap_t *other);
 
 /** @brief Splits the bitmap by assigning all bits starting at \b offset to the
  * \b other bitmap while removing them from \b map.
diff --git a/src/sparsemap.c b/src/sparsemap.c
index 10ed7a6..0520da9 100644
--- a/src/sparsemap.c
+++ b/src/sparsemap.c
@@ -821,6 +821,7 @@ __sm_get_size_impl(sparsemap_t *map)
   return SM_SIZEOF_OVERHEAD + p - start;
 }
 
+#ifdef DIAGNOSTIC
 /** @brief Aligns to SM_BITS_PER_VECTOR a given index \b idx.
  *
  * @param[in] idx The index to align.
@@ -832,6 +833,7 @@ __sm_get_aligned_offset(size_t idx)
   const size_t capacity = SM_BITS_PER_VECTOR;
   return (idx / capacity) * capacity;
 }
+#endif
 
 /** @brief Aligns to SM_CHUNK_MAP_CAPACITY a given index \b idx.
  *
@@ -1264,13 +1266,20 @@ sparsemap_scan(sparsemap_t *map, void (*scanner)(sm_idx_t[], size_t, void *aux),
   }
 }
 
-void
+int
 sparsemap_merge(sparsemap_t *map, sparsemap_t *other)
 {
   uint8_t *src, *dst;
-  size_t src_count = __sm_get_chunk_map_count(other), dst_count = __sm_get_chunk_map_count(map), max_chunk_count = src_count + dst_count;
+  size_t src_count = __sm_get_chunk_map_count(other);
+  size_t dst_count = __sm_get_chunk_map_count(map);
+  size_t max_chunk_count = src_count + dst_count;
+
+  /* Estimate worst-case overhead required for merge. */
+  if (map->m_data_used + src_count * (sizeof(sm_idx_t) + sizeof(sm_bitvec_t) * 2) > map->m_capacity) {
+    errno = ENOSPC;
+    return -1;
+  }
 
-  // TODO: ensure there is space, or ENOSPC
   dst = __sm_get_chunk_map_data(map, 0);
   src = __sm_get_chunk_map_data(other, 0);
   for (size_t i = 0; i < max_chunk_count && src_count; i++) {
@@ -1321,6 +1330,7 @@ sparsemap_merge(sparsemap_t *map, sparsemap_t *other)
       src_count--;
     }
   }
+  return 0;
 }
 
 void